Within a nuclear reactor, the upper boundary of the reactor core is defined by the upper core plate. The upper ends of the nuclear fuel assemblies that heat the water circulating through the core are detachably engaged the undersurface of the upper core plate. Each fuel assembly includes an array of spaced-apart fuel rods within which nuclear fuel pellets are disposed. Each fuel assembly further has a plurality of tubes disposed between the fuel rods that receive neutron-absorbing nuclear reactor control rods for controlling the power output of the fuel assemblies and the reactor core. Reciprocal movement of the nuclear reactor control rods within these tubes is implemented by control rod drive mechanisms (CRDM's) through drive rods that extend through the pressure vessel.
Such nuclear reactors further include an upper support plate that is vertically spaced above the upper core plate. An upper plenum chamber is defined between the upper support plate and the upper core plate. Reciprocal movement of the nuclear reactor control rods occurs within the upper plenum chamber where the drive rods are connected to the upper end of the control rods. In this plenum chamber, the control rods and their respective drive rods are slidably received within guide tubes which are interposed-between, and connected to, the upper surface of the upper core plate and the upper support plate. These guide tubes properly align the control rods with their respective fuel assemblies, and also serve to prevent control rods and drive rods from laterally vibrating from cross-currents present in the surrounding, rapidly circulating coolant. Annular flanges are provided at the lower ends of the guide tubes as part of a mechanism for securing them to the upper core plate.
Guide tube retaining pins mount the guide tube flanges around an opening in the upper core plate. These pins are vertically disposed, and have lower portions called leaves which are laterally resilient and frictionally engageable within suitable bores present in the upper core plate. The upper portion of each pin is inserted into holes in the guide tube lower flange to form an interference fit and is subsequently locked in place onto the flange with a locking system comprising two small welds.
The purpose of such guide tube retaining pins is to mount the nuclear reactor control rod guide tubes within the upper core plate in such a way that lateral, vibrating motion is resisted while vertical movement caused by differential thermal expansion is tolerated. However, in some reactors, bending stresses caused by, for example, a non-perpendicular orientation between the pins and the guide tube flange may cause stress corrosion cracking problems to develop within the retaining pins and weaken them to a point where they must be repaired or replaced by a welding operation. Because the retaining pin and system which locks it in place on the guide tube flange are located within operating plants in an irradiated, underwater environment, the welding operations are difficult to achieve. The small size of the retaining pin and locking system, and the confined area within which the welding operations must be performed contribute to this difficulty. Furthermore, the heat generated by the welding operation itself causes other difficulties to arise, such as the sensitization of structural material adjacent to the welded area to stress corrosion cracking. Finally, even remotely operated underwater welding operations can expose workers to potentially dangerous radiation.
A dual crimp locking system is disclosed in commonly-assigned U.S. Pat. No. 4,770,846 to Land, Hopkins, and Martinez, the disclosure of which is incorporated by reference. The '846 patent discloses a support pin having a split-leaf base section, an externally threaded, upper bolt portion, and a top end portion provided with vertical grooves. The grooved top end protrudes through a securing nut threaded onto the upper bolt portion. The nut is provided with vertical splines and the locking system is secured by a stepped tubular cap which is crimped into place around both the top end portion of the support pin and the securing nut to positively prevent retrograde rotation between the support pin and nut. This fastening arrangement depends for its efficacy on the structural integrity of the crimped cap, i.e., the stepped tubular cap, as well as that of the support pin. However, the structural configuration of these support pins subjects them to stresses which may promote cracking.
If the retaining pin and locking system should fail, the longevity of the nuclear reactor as a whole may be seriously compromised by the resulting dislodged parts that would be propelled through the system by the swiftly flowing coolant. These dislodged parts may damage other power plant components, such as the heat exchanger tubes in the steam generators. If the shanks of prior art support pins should fail from stress corrosion cracking, the upper bolt portion of the support pin, along with the attached nut or crimped cap, may subsequently dislodge under the influence of the coolant flow.
These concerns were addressed in the commonly assigned U.S. Pat. No. 4,772,448 to Popalis, Hopkins, Land, and Obermeyer, the disclosure of which is also incorporated by reference. The '448 patent provides a mechanical support pin system using a split leaf support pin to mount the control rod guide tubes to the upper core plate. The support pin has a first pin portion that is insertable through a bore in the guide tube flange, and a second pin portion fixedly secured within the bore and having a solid body section and a split-leaf base section. The solid body section has an outer diameter that is accommodated in the bore by a close clearance fit, and the split-leaf base section has a split intermediate section and a split-end section. The split intermediate section extends from the solid body section and has an outer diameter that is smaller than the outer diameter of the solid body section. The split end section extends from the split intermediate section and biasingly engages at least a portion of the wall of the bore. The support pin is secured within the second structural member by a frictional fit and loads applied transversely to the longitudinal axis of the support pin are reacted substantially in pure shear by the second pin portion substantially through the solid body section. A locking nut threadedly engages an externally threaded section of the first pin portion and cooperates therewith. This support pin system also includes a locking nut retainer having a crimpable split cylindrical wall portion and an axial slot and tabs that extend radially from the wall portion along its external surface. The locking nut retainer is positioned around at least a portion of the locking nut. The guide tube flange has a counter bore having an annular recess radially defined in the wall to accommodate at least the portion of the wall portion of the locking nut retainer which includes the tabs. The tabs are positioned within the annular recess and the locking nut retainer is positively retained in the guide tube flange. The locking nut may further include a crimp receiving section having at least one recess in its external surface, so that a portion of the wall portion of the locking nut retainer crimpingly engages the recess to positively retain the locking nut in position around the support pin and prevent the nut from becoming a loose piece if the support pin breaks in the shank region.
While the support pin system of the '448 patent is a substantial advance in the art, it unfortunately does not overcome all of the problems of the prior art. Although the split intermediate section of the second pin portion has an outer diameter less than the outer diameter of the solid body section so that transverse loads are reacted substantially in pure shear by the second pin portion through the solid body section, the shear stresses under certain emergency operating circumstances may come close to exceeding the yield point for the support pin. Additionally, the use of the locking nut retainer around the locking nut requires a perpendicular relationship between the guide tube flange and the locking nut if unwanted loads are to be avoided. Where these two components are skewed rather than perpendicular, unwanted bending loads are imposed on the shank of the support pin when the locking nut is tightened.
Clearly, there is a need for a supporting pin system in which the stresses in the support pin will not exceed the yield strength of the pin under any type of operating circumstances and in which the locking nut will not impose bending loads on the support pin.